1. Field of the Invention
The present invention relates to an optical disc drive, and more particularly, to an optical disc drive capable of effectively preventing a damage of an optical disc by restraining a growth of a crack generated at an inner circumferential region of the optical disc.
2. Description of the Conventional Art
Generally, a compact disc for recording data including an audio signal or a video signal by a digital recording method, and a digital versatile disc having an increased recording capacity and using a red semiconductor laser of a short wavelength are called as an optical disc.
FIG. 1 is a plane view showing a general optical disc, and FIGS. 2A to 2C are views showing a growth process of a crack generated at an inner circumferential region of the optical disc.
As shown, a general optical disc 10 includes: a hole 10a formed at the center thereof; an inner circumferential region 11 formed at a circumference of the hole 10a and having no recording film; and an outer circumferential region 12 formed at a circumference of the inner circumferential region 11.
A crack generated at the inner circumferential region of the optical disc is caused by a user's carelessness, or is caused as a material of the optical disc is composed of polycarbonate, an amorphous material.
The optical disc formed of the polycarbonate, an amorphous material is damaged when even a stress corresponding to 1/10 of a yield stress is continuously applied thereto at an ordinary temperature.
Generally, the optical disc is damaged by a following process. At the time of fabricating the optical disc or dealing with the optical disc by a user, a minute scratch is generated as a crack at the periphery of the hole of the optical disc. The scratch is gradually increased by a craze phenomenon thereby to damage the optical disc.
The craze phenomenon is a phenomenon that a chain structure inside the amorphous material is gradually cut to be destroyed and thereby a crack gradually grows.
That is, the amorphous material such as polycarbonate has a chain structure. As shown in FIG. 2, when a minute crack of a sub-micron unit is initially generated, the chain structure is gradually scattered. Under this state, when a static fatigue is applied to the amorphous material, a distance of each molecule is drastically increased from ‘d’ of FIG. 2A to ‘le’ of FIG. 2B thereby to destroy a bonding state between molecules and to generate a void space between the craze and the molecule more and more. Under this state, when the static fatigue is continuously applied to the amorphous material, the chain structure is cut as shown in FIG. 2C and the crack gradually grows thereby to damage the amorphous material. This phenomenon is called as a craze phenomenon.
The reference signal a of FIGS. 2A to 2C signifies a size and a direction of a stress, which shows that the crack size is increased as the craze phenomenon becomes severe when a load more than a certain degree is constantly applied.
In the optical disc fast rotated in the optical disc drive, a minute scratch generated at the periphery of the hole develops as a crack by the aforementioned craze phenomenon, and the crack gradually grows thereby to damage the optical disc. According to this, information stored in the optical disc is lost and the optical disc drive itself is also damaged.
As a recent optical disc drive, a 52-speed recording reproducing apparatus operated at least 10,000 rpm is generally used. As a DVD speed is constantly increased, the damage of the optical disc due to the high-speed of the optical disc drive has to be prevented. In case of the 52-speed optical disc drive, the damage of the optical disc is generated.
As one embodiment of the conventional art for preventing a damage of an optical disc, there is a disc type recording medium disclosed in Korean Open-Laid Publication Patent No. 2002-0049733.
According to the disc type recording medium disclosed in Korean Open-Laid Publication Patent No. 2002-0049733, a ring type protrusion portion is formed at the periphery of the hole of the optical disc in order to prevent a crack growth when the optical disc is rotated. However, a width and a height of the protrusion portion are limited according to a size of a space inside the optical disc drive.
As another embodiment of the conventional art for preventing a damage of an optical disc, there is a structure for preventing a damage of a compact disc disclosed in Korean Open-Laid Publication Patent No. 2001-0091302.
According to the structure for preventing a damage of a disc type recording medium disclosed in Korean Open-Laid Publication Patent No. 2001-0091302, a metal reinforcing member of a non-magnetic substance is attached to the periphery of the hole of the optical disc. However, due to an elastic coefficient difference between the polycarbonate, the material of the optical material and the metal reinforcing member, a stress is concentrated at a contact part of the metal reinforcing member and thereby the metal reinforcing member is easily detached from the optical disc.